Place the chicken, stock, fish sauce and grated ginger in a large saucepan and bring to the boil, then cover and simmer for 10-15 minutes until the chicken is cooked through.
Meanwhile, boil water and soak the noodles in boiling water according to the packet instructions.
Drain the noodles and divide them between four serving bowls. Spoon the chicken, beansprouts and onions over the top, then sprinkle with the basil, coriander and mint. Drench the mixture with the chicken broth, then scatter the chillies and spring onions over the top.